At each leaf or ASBR, there are two FECs: a lower FEC and an upper FEC. The lower FEC is used for hashing to multiple ASBRs and the upper FEC is used to choose the next-hop that connects the leaf node to the ASBR. Hashing is performed based on the opaque value of the FEC. See the ‟Supported Recursive Opaque Values” section in the 7450 ESS, 7750 SR, 7950 XRS, and VSR MPLS Guide for more information.
In Figure: Hashing for inter-AS, the leaf generates a lower FEC <0.0.0.0, opaque <10.0.0.14, 8010>>. The lower FEC’s opaque <10.0.0.14, 8010> and number of ASBRs (three) are used to decide which ASBR is used based on hashing. After hashing produces ASBR-5 as the result, the upper FEC of <10.0.0.23, opaque <10.0.0.14, 8010>> is created. This upper FEC is used to resolve the ASBR-5 next-hop between the three interfaces that connect the leaf node to ASBR-5.
